<!DOCTYPE html>
<html lang="zh-CN">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, user-scalable=no, initial-scale=1.0, maximum-scale=1.0, minimum-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style>
        body {
            background: #000;
            text-align: center
        }

        #c1 {
            background: #fff;
        }
    </style>

</head>

<body>
    <canvas id="c1" width="800" height="600">

    </canvas>
</body>
<script>
    let oC = document.getElementById('c1');
    let gd = oC.getContext('2d');       //图形上下文-绘图用的接口
    //arc(圆心X，圆心Y，半径,其实角度，结束角度,是否逆时针)
    //arc(cx,cy,r，startAng,endAng，是否逆时针)
  
    // 角度   360度
    // 弧度   2PI弧度
    // 360度=2PI弧度
    // 180 = pI
  
  
    gd.arc(400,300,150,0*Math.PI/180,90*Math.PI/180,true)
    gd.stroke()
</script>

</html>